Soot formation can be a problem in diesel engines since the engine environment promotes soot formation, accumulation, and agglomeration. The degree of soot formation depends on design and operating parameters. Problems result in the engine when the soot particles, which are formed in the combustion chamber, and any adsorbed species aggregate to form larger particles which increase system viscosities. Eventually, an engine oil may become so viscous that it cannot be pumped, resulting in engine failure. In addition, hardware design changes have been made in diesel engines to meet emissions requirements and improve environmental performance. These changes have resulted in more soot being diverted to the crankcase and, therefore, certain newer engines have experienced unacceptable viscosity increase.
Therefore, improved lubricants which perform well in a variety of engine types and varying engine conditions and which have better soot handling capabilities are needed. Accordingly, development of a desired lubricant may require extensive engine tests to determine success. However, the tests used to evaluate how a given lubricant performs, e.g., tests which evaluate soot-related viscosity increase in diesel engines, are very expensive, time consuming, and require a large amount of test sample. In addition, testing may be hindered by test stand availability. Determining crucial parameters for lubricant formulations by matrixing designed experiments, therefore, may be prohibitive.
As a result, there has been a clear need in the industry for a bench test which correlates well with standardized engine tests, for example, the Mack Truck Technical Services Standard Test Procedure No. 5GT 57 entitled "Mack T-7: Diesel Engine Oil Viscosity Evaluation," dated Aug. 31, 1984 ("Mack T-7") and the Mack Truck Technical Services Standard Test Procedure entitled "Mack T-8: Diesel Engine Oil Viscosity Evaluation," dated October 1993 ("Mack T-8"). Several bench test designs have attempted to create simulated soot loading, but published tests have not achieved stable simulated soot loading or successful correlation with standardized engine tests and, at the same time, reproducibility.
One test entailed diluting a used oil and observing the resulting viscosity. The presumption was that a good oil will result in a lower overall viscosity. However, this technique was unsuccessful because of the strong non-Newtonian nature of thickened oils. For example, addition of a fresh oil to a used oil can produce as much as a 20% viscosity decrease which masks any generated viscosity interaction when different fresh oils are tested and also indicates that oil additions just prior to any viscosity measurement will drastically alter results.
Because it was recognized that soot played a role in the thickening of engine oils, bench test development using carbon black was initiated. In one bench test, carbon black was dispersed into fresh engine oils using sonic shear and the viscosity response was measured. This technique was first tested using an ultrasonic bath and then a high power sonicator, e.g., SONICATOR W-375. However, the carbon black in the dispersions would gradually precipitate when left standing in contrast to used oil samples which have the ability to maintain their soot loading in suspension. This method of creating a carbon dispersion by sonic shear does not sufficiently mimic an engine environment.
Direct dispersion, which is another bench test attempt, requires dispersing carbon black directly into a sample. For example, in Chevron's soot-thickening test method LPTL 2007A, a specified amount of carbon black can be blended into an oil and the time for a volume of the oil to flow through a calibrated glass capillary viscometer is measured. In another bench test method, carbon black has been directly dispersed into a sample using a high-speed blender followed by agitation at elevated temperatures. "Fluffy" or low density carbon black was used, although it is awkward to handle, because the fluffy carbon black can be dispersed directly by the high-speed blender. However, these methods appear to be unsuccessful because the direct dispersion systems rapidly precipitated. Consequently, these systems do not accurately simulate soot loading and, therefore, fail to accurately recreate an engine test environment.
This invention seeks to provide a solution to the deficiencies in previous bench test systems by providing a bench test that simulates the physical effects of soot-loading in standardized engine tests and generates results which are reproducible and correlate well with the engine tests. In particular, the bench test method of this invention creates a carbon black agglomerate size which is an order of magnitude smaller than in the previous bench tests using direct dispersions and which is more closely related to the size of soot particles in used oils.